The mammalian midbody is an enigmatic structure in that while it is a constant feature of mitosis and cytokinesis its exact relationship to these important cellular events is not understood. The proposed work will be directed toward answering some of the questions regarding the composition of the midbody and its possible functional roles in cell division through the use of large populations of synchronized cultured cells. Composition of the midbody, particularly of the dense matrix components, will be investigated. Distribution of midbody components prior to late anaphase, when they are first seen with electron microscopy, will be studied and interpreted in terms of mitotic function. Specific associations between the midbody and its overlying plasma membrane will also be studied to assess further the likely functional role for the midbody in the terminal phase of cytokinesis.